This invention relates to optical objective lens systems having selectively shiftable equivalent focal lengths, and more particularly to an optical objective of the zoom type adapted for use with a plurality of attachment lenses of different magnification and comprising a zoom control front assembly and a rear stationary rear assembly which includes an intrinsic exchangeable front part arranged upon substitution of one of the attachment lenses to shift the range of variation of magnification of the master lens toward longer equivalent focal lengths.
U.S patent application Ser. No. 604,484, now U.S. Pat. No. 4,015,895, assigned to the same assignee as this application, discloses a zoom lens adapted for use with an afocal attachment lens to be inserted between the front and rear parts of the image-forming lens. The present invention relates to an improvement of the lens system of the application Ser. No. 604,484 to achieve a significant shortening in the size of the image-forming lens along the optical axis thereof.